The magnitude-8.9 earthquake that struck northern Japan on Friday not only violently shook the ground and generated a devastating tsunami, it also moved the coastline and changed the balance of the planet.

Meanwhile, NASA scientists calculated that the redistribution of mass by the earthquake might have shortened the day by a couple of millionths of a second and tilted the Earth’s axis slightly.

Not all of Japan jumped 13 feet closer to the United States, said Kenneth W. Hudnut, a geophysicist with the United States Geological Survey. The shifts occurred mostly in the area closest to the epicenter, and stations farther away reported much less movement.

That part of Asia, to the surprise of many who look at the geological map, sits on the North American tectonic plate, which wraps up and around the Pacific plate and extends a tentacle southward that part of Japan sits atop. The Pacific plate is moving about 3.5 inches a year in a west-northwest direction, and in that collision — what geologists call a subduction zone — the Pacific plate dives under the North American plate.

Most of the time, the two tectonic plates are stuck together, and the North American plate is squeezed, much like a playing card held between the thumb and forefinger.

As the fingers squeeze the card, it buckles upward until the card pops free.

In the same way, the North American plate buckles, and the eastern part of Japan is slowly pushed to the west. But when the earthquake, which occurred offshore, released the tension, the land jumped back to the east.

As it unbuckled, a 250-mile-long coastal section of Japan dropped in altitude by two feet, which allowed the tsunami to travel farther and faster onto land, Dr. Stein said.

On a larger scale, the unbuckling and shifting moved the planet’s mass, on average, closer to its center, and just as a figure skater who spins faster when drawing the arms closer, the Earth’s rotation speeds up. Richard S. Gross, a scientist at NASA’s Jet Propulsion Laboratory, calculated that the length of the day was shortened by 1.8 millionths of a second.

The earthquake also shifted the so-called figure axis of the Earth, which is the axis that the Earth’s mass is balanced around. Dr. Gross said his calculations indicated a shift of 6.5 inches in where the figure axis intersects the surface of the planet. That figure axis is near, but does not quite align with, the rotational axis that the Earth spins around.

Earlier great earthquakes also changed the axis and shortened the day. The magnitude-8.8 earthquake in Chile last year shortened the day by 1.26 millionths of a second and moved the axis by about three inches, while the Sumatra earthquake in 2004 shortened the day by 6.8 millionths of a second, Dr. Gross said.

Such changes are not unusual, and even without earthquakes, changes in ocean currents and atmospheric conditions usually have even greater effects. “The Earth is always wobbling, and the length of the day is always changing,” Dr. Gross said.

What is perhaps most surprising about the Japan earthquake is how misleading history can be. In the past 300 years, no earthquake nearly that large — nothing larger than magnitude-eight — had struck in the Japan subduction zone. That, in turn, led to assumptions about how large a tsunami might strike the coast.

“It did them a giant disservice,” said Dr. Stein of the geological survey. That is not the first time that the earthquake potential of a fault has been underestimated. Most geophysicists did not think the Sumatra fault could generate a magnitude-9.1 earthquake, and a magnitude-7.3 earthquake in Landers, Calif., in 1992 also caught earthquake experts by surprise.

“Perhaps the message is we should re-evaluate the occurrence of superlarge earthquakes on any fault,” Dr. Stein said
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